This invention is directed to a method for operating an oxygen blown steelmaking furnace, and in particular, it is directed to a method for cooling the liquid steel contained in the furnace by blowing nitrogen gas into the bath after the oxygen blow is discontinued.
Steelmaking may be regarded as a rapid oxidation process where iron is refined to produce a high-purity low-carbon liquid steel and molten slag that contains a combination of oxides. The rapid oxidation of the residual elements in the metal provides more than enough heat to produce the liquid steel product and to melt any fluxes that are added to the slag. In many instances, excess heat is generated from the oxidation process, the excess heat raising the liquid steel temperature above a desired tapping temperature. In such cases, the liquid steel must be cooled down to the aim or target temperature before it can be tapped for further processing downstream.
In the past, steelmakers reduced the temperature of their liquid steel by periodically charging additional amounts of cold scrap, iron ores, and/or other coolants such as dolomitic stone, limestone, and revert materials, into the steelmaking furnace. To illustrate, U.S. Pat. No. 6,136,066 granted to Lynn, et al. teaches that during the reaction, temperatures within the furnace are elevated as the blown oxygen removes carbon from the bath. Iron ore is charged during the oxygen blow to provide a cooling affect on the reaction, and the ore charge prevents temperatures from reaching undesirable levels. Lynn, et al. also teaches that a slag/sludge revert produced by the steps of their present invention, in the form of briquettes or granular material, may be blended with the iron ore and charged as a coolant during the oxygen blow.
U.S. Pat. No. 5,135,572 to baraki, et al. also teaches feeding iron ore as a coolant in a steelmaking process. In the examples provided by the inventors, ore, having a particle size distribution as indicated in the tables of the patent, is supplied during the blows as a coolant.
Charging such bulk materials as iron ore and reverts into a steelmaking furnace requires charging systems similar to the drawing figures shown in the Ibaraki, et al. patent. Such systems add installation, logistical, and maintenance costs to a steelmaking operation. For example, complex hopper distribution systems are expensive to install and maintain. Supplying a steelmaking operation with bulk coolant materials is both transportation and labor intensive, and bulk materials require large storage facilities. Finally, purchasing iron ores for use as a coolant further increases costs associated with a steelmaking operation.
It is therefore an object of the present invention to provide an alternative to charging bulk coolant materials into an oxygen blown steelmaking furnace to achieve aim or target liquid steel temperature.
It is a further object of the present invention to provide an improved method for operating an oxygen blown steelmaking furnace to achieve a liquid steel target temperature.
It is another object of the present invention to provide a method for introducing a coolant into liquid steel manufactured in an oxygen blown steelmaking furnace.
It is an another object of the present invention to blow a cooling gas into a finished molten steel bath whereby the cooling gas provides additional stirring and effective mixing at the molten slag/steel interface to promote removal of impurities such as sulfur and phosphorus from the molten steel.
And finally, it is still another object of the present invention to blow a cooling gas into a finished molten steel bath whereby the blown cooling gas provides additional stirring at the molten slag/steel interface and thereby reduces oxygen levels in the molten steel.
In satisfaction of the foregoing objects and advantages, the present invention provides a method for operating an oxygen blown steelmaking furnace to achieve target temperature in the liquid steel being manufactured therein, the steps of the method include blowing nitrogen gas into the liquid steel after the oxygen blow is completed. The nitrogen gas is blown through the main lance into the liquid steel for a selected time and selected flow rate based upon particular steelmaking conditions, the selected time and selected flow being sufficient to reduce the liquid steel to a target temperature. The nitrogen gas blow into the liquid steel is discontinued when the selected time is reached.
In satisfaction of the foregoing objects and advantages, the present invention provides a method for operating an oxygen blown steelmaking furnace to achieve target temperature in the liquid steel being manufactured therein, the steps of the method include injecting nitrogen gas into the liquid steel after the oxygen blow is completed. The nitrogen gas is typically injected through the main lance or bottom tuyeres into the liquid steel for a selected time and selected flow rate based upon particular steelmaking conditions, the selected time and selected flow being sufficient to reduce the liquid steel to a target temperature. The nitrogen gas blow into the liquid steel is discontinued when the selected time is reached.